Circulatory blood flow delivers oxygen and nutrients to tissues and organs and removes toxins and wastes therefrom. Such delivery and removal is essential to maintaining cellular function and tissue and organ health. Broadly defined, stress is the aggregate impact of physical, cognitive, pathological, and environmental factors to which an organism must adapt in order to remain in a physiologically homeostatic state. Adequate circulatory blood volume must be maintained under varying forms and degrees of stress, or else homeostasis and adequacy of oxygenated blood flow delivery is compromised. Accordingly, in the healthy state, the autonomic nervous system continuously adjusts circulatory blood volume in order to meet these constantly changing demands. In situations where the ability to adjust circulatory blood volume is inadequate, the delivery of oxygen and nutrients to tissues and organs and the removal of toxins and wastes therefrom is inadequate to meet the cellular demands and, as a result, overall physiological function is compromised.
Systems and methods for evaluating the condition of the autoregulatory components of the cardiovascular system are known in the art. Unfortunately, while these systems and methods are good predictors of the overall cardiovascular condition resulting from long-term pathological and age-related structural changes, they cannot characterize the functional adequacy of circulatory blood volume in the short-term. As such, in the face of stress, any resultant deficiencies in supplying the demands of the tissue and organs is often not detected until physiological function is so compromised that tissue and organ dysfunction become symptomatic and sustainability is at risk. Furthermore, while levels of certain metabolites are indicative of inadequate circulatory blood volume, such metabolites are only present after prolonged inadequate circulatory blood volume has occurred and therefore cannot characterize the functional adequacy of circulatory blood volume in the pre-symptomatic stages to avoid a compromised physiological state that may be irreversible. Thus, there is a need for real-time systems and methods that characterize the adequacy of circulatory blood volume over contiguous, finite time intervals in order that circulatory blood volume may be assessed and any deficiencies in supply may be detected and treated before the patient's sustainability is at risk.